Interpretive Summary: The Louisiana sugarcane industry is heavily dependent upon a single variety, LCP 85-384, which occupies over 85% of the sugarcane land. LCP 85-384 is highly resistant to mosaic, a serious disease of sugarcane caused by a virus; however, if a new strain of the virus were to emerge that can overcome the resistance to the disease, the disease could move rapidly through the industry and cause significant yields losses. The researchers have examined close to 400 individual isolates of the mosaic virus to determine what strains of the virus are present in Louisiana. Results suggest that a previously minor strain of the virus has become the predominant strain in the virus population and a new strain has appeared in some varieties of sugarcane. The new strain does not appear to be able to attack LCP 85-384, but its appearance does demonstrate that the virus is still capable of change and may be responsible for the loss of some potential new varieties. The emergence of a minor strain as the predominant strain since the virus population was last studied in the mid 1990s may explain why some experimental varieties nearing the end of their 14 years of testing have become susceptible to mosaic and have had to be eliminated. As researchers select parents for the next generation of sugarcane varieties, they need to insure that the parent varieties used are resistant to the different strains of the mosaic virus.

Technical Abstract:
Leaf samples from 390 sugarcane plants showing mosaic symptoms were collected in 2001 and 2002 at 9 locations within the Louisiana sugarcane industry. Virus isolates associated with the diseased plants were identified using reverse transcriptase-polymerase chain reaction (RT-PCR) to distinguish between sugarcane mosaic virus (SCMV) and sorghum mosaic virus (SrMV). A restriction fragment length polymorphism (RFLP) analysis of the RT-PCR product was then used to identify strains within each virus. SrMV strains I, H, and M were associated with 70%, 9%, and 2% of the plants with mosaic symptoms, respectively. In previous surveys conducted between 1978 and 1995, over 90% of the plants sampled were infected with SrMV strain H. The remaining 10% was mostly infected with SrMV strain I, except for an occasional sample with SrMV strain M. No SCMV strain was associated with any diseased plant collected during the 1978-1995 or the 2001-2002 surveys. RT-PCR showed that approximately 14% of the samples collected in 2001-2002 were infected with SrMV, but the RFLP banding pattern did not match any described strain. No RT-PCR product was produced by either the SCMV- or the SrMV-specific RT-PCR primer set for 5% of the plants showing mosaic symptoms suggesting that another virus may cause mosaic in sugarcane in Louisiana.